Semiconductor device with a semiconductor element provided in a mesa structure

Abstract: The invention relates to a semiconductor device with a semiconductor body (1) comprising a semiconductor element with connection points (2, 3) which adjoins a surface (4) of the semiconductor body (1) and is laterally insulated and surrounded by a first depression (5) in the surface (4), which depression (5) is provided with a wall (6) and a bottom (7), while the surface (4) of the semiconductor body (1) and the wall (6) and bottom (7) of the depression (5) are covered with an insulating layer (8), the connection points (2, 3) being provided in the insulating layer (8) on the surface (4) of the semiconductor body (1) and being connected to conductor tracks (10, 11) which connect the connection points (2, 3) across a wall (6) to connection surfaces (12, 13) associated with the connection points (2, 3) and situated on the bottom (7). It is found in practice that, in the case of a progressive miniaturization, the manufacture of such devices leads to rejects caused by short-circuits between connection surfaces (12, 13). According to the invention, one or several further depressions (50) surrounded by the first depression (5) is/are present in the surfce 4 of the semiconductor body (1), while only one connection surface (12, 13) is present on the bottom (7, 57) of each of the first and further depressions (5, 50), which connection surface is connected to a connection point (2, 3) on the surface (4) of the semiconductor body (1) via a conductor track (10, 11). Short-circuits are found to take place in the known semiconductor devices on a wall (6) of the depression (5) when there are more than one conductor tracks (10, 11) on this wall (6). Thanks to the measure according to the invention, only one conductor track (10 or 11) is present on the wall (6, 56) of the depression (5, 50), so short-circuits with other conductor tracks on the wall (6, 56) cannot arise.

Description: The present invention relates to a semiconductor device as defined in the pre characterizing part of claim 1. Such a device is also known as a semiconductor device with mesa insulation. The semiconductor element for example is a bipolar transistor whereby the semiconductor zones provided in the upper surface of the mesa are the base- and the emitterzone of the transistor. These semiconductor zones are connected to bond pads by means of conductor tracks provided on the insulating layer. In this example the emitterzones are connected to an emitter bond pad, the base zone to a base bond pad. The bond pads serve to make electrical contact with connection legs of a lead frame, for example, through bond wires or contact elevations, so-called bumps. The bond pads are comparatively large in order to facilitate such a contact. The bond pads accordingly are present next to the mesa on the surface of the semiconductor substrate and not on top of the mesa. Placement of comparatively large bond pads on top of the mesas would lead to an enlargement of the mesa, which leads to high capacitances in the element. This may render the element unsuitable for high frequencies. Patent Abstracts of Japan vol. 16, no. 330 [4-199719] discloses a device of the kind mentioned before with a high electron mobility transistor (HEMT) as a semiconductor element. This element comprises a gate-, source - and drainzone formed in the upper surface of the mesa. These zones are connected through conductor tracks to bond pads on the surface of the substrate next to the mesa. In practice, semiconductor devices with ever smaller details are manufactured owing to an increasing miniaturization. The known semiconductor device has the disadvantage that the manufacture of miniaturized known semiconductor bodies leads to rejects owing to short-circuits between bond pads of the semiconductor element. The invention has for its object inter alia to design the semiconductor device mentioned in the opening paragraph in such a manner that the said disadvantage does not occur. According to the invention, the device is for this purpose characterized by the features mentioned in the characterizing part of claim 1. It is found that short-circuits between the conductor tracks which connect the semiconductor zones to the bond pads may occur in the known device. The invention is based on the recognition that these short-circuits take place when more than one conductor track is present on the wall of the mesa. The short-circuit is found to take place between the tracks on that wall. Especially the formation of the conductor tracks on the wall through patterning of a photoresist and etching of a conducting layer, for example, by reactive ion etching (RIE), gives rise to short-circuits. When the illumination of the photoresist or etching of the conducting layer takes place imperfectly owing to a comparatively steep wall of the mesa, traces of the conducting layer remain between the tracks on the wall, whereby these conductor tracks can be short-circuited. Often a continuous conducting layer is created then all around the mesa on the wall. The measure according to the invention means that there is only one connection to be formed on each wall of the mesa. There is only one conductor track on each wall of the mesa, so short-circuiting with other tracks cannot occur. Since the walls are fully separated from one another, i.e. have no connections to one another, short-circuiting between conductor tracks is not possible. It is noted that it is possible for a semiconductor element, for example a transistor, to be provided with several separate emitter- and base regions. In such a situation the separate emitter regions may be connected to one emitter bond pad and the separate base regions to one base bond pad. The conductor tracks may be manufactured from various materials such as aluminium or polysilicon. In a preferred embodiment of the invention, the conductor tracks comprise gold. Conductor tracks comprising gold have a low resistance, which renders possible high switching frequencies of the semiconductor element. When the bond pad is also made from an alloy comprising gold, there will be no corrosion problems between bond pads and gold bond wires or bumps such as those which are widely used for making connections between bond pads and a connection leg of the housing. The very nature of the noble metal gold implies that layers comprising gold have comparatively bad etching properties, so that an increasing miniaturization in the known device may readily lead to short-circuits between conductor tracks. Such problems do not arise in a device according to the invention. In a further embodiment, the wall of the mesa and the wall of the depression enclose an angle of more than 50° with the surface of the substrate. This embodiment has the advantage that the mesa structure is sharply defined, whereby parasitic capacitances in the semiconductor element remain small. The device according to the invention gives no problems involving short-circuiting of conductor tracks even at such an acute angle between the wall and the bottom. In an embodiment of the device, the conductor track is patterned only on the surface of the mesa, the surface of the substrate next to the mesa and the bottom of the depression. The conductor tracks on the walls of the mesa are not patterned then. The walls are entirely covered with a conducting layer. Owing to the special geometry of the semiconductor device, no short-circuiting between bond pads occurs even in that case. In a further embodiment of the invention, a further bond pad is present on the surface of the substrate next to the mesa. This further bond pad may be, for example, a connection for a collector of a bipolar transistor. Two bond pads are then present on the surface of the substrate next to the mesa, i.e. one connected to a track leading to the top of the mesa and connected for example to the semiconductor substrate. In spite of the fact that there are two bond pads present on the surface of the substrate next to the mesa, only one conductor track is present on the wall of the mesa. No short-circuiting can accordingly occur between the two bond pads. The invention will be explained in more detail below by way of example with reference to the drawings, in which: Fig. 1 is a plan view of a known semiconductor device with a transistor in a mesa structure; Fig. 2 is a cross-section taken on the line I-I in Fig. 1 of a known semiconductor device; Fig. 3 is a plan view of a semiconductor device with a transistor in a mesa structure according to the invention; Fig. 4 is a cross-section taken on the line II-II in Fig. 3 of a semiconductor device according to the invention; Fig. 5 is a plan view of a semiconductor device with a number of semiconductor sub-elements according to the invention; Fig. 6 is a cross-section taken on the line III-III in Fig. 5 of a semiconductor device according to the invention; and Fig. 7 is a cross-section taken on the line IV-IV in Fig. 5 of a semiconductor device according to the invention. The Figures are purely diagrammatic and not drawn to scale. Corresponding parts have generally been given the same reference numerals in the Figures. Figs. 1 and 2 show a known semiconductor device with a semiconductor body 1 consisting of a semiconductor substrate 21 and a mesa 22 formed on the semiconductor substrate 21, said mesa having an upper surface 4 and a wall 6. In this example, the device comprises an n + -type silicon substrate 21 provided with an n - -type epitaxially grown layer 22. The substrate 21 and the layer 22 act as the collector region of the transistor. A p-type layer 23 which acts as the base region is provided in this layer 22 through implantation and diffusion. The base region 23 of the transistor is provided in the upper surface 4 of the mesa 22 with a semiconductor zone 25 with a p + -type doping for contacting the base region 23. An n + -type emitter region 24 is also provided in this base region 23 in the upper surface 4. The layer 22 and the base region 23 are laterally bounded by the depression 5. Edges of the depression are indicated with dotted lines 14, 15 (Fig. 1) in plan view. An insulating layer 8 is formed on the upper surface 4 and the wall 6 of the mesa 22, on the surface 7 of the substrate 21 next to the mesa 22. In this example made of silicon oxide. Contact holes 2,3 are formed in the insulating layer 8 on the upper surface 4 of the mesa 22 in which semiconductor zones 24,25 of the semiconductor element are exposed. The base and emitter zones are electrically connected to base and emitter bond pads 12, 13 by means of conductor tracks 10, 11, which are usually made of the same material as the bond pads and are provided on the insulating layer 8. The bond pads 12, 13 serve for making electrical contact with connection legs of a lead frame by means of, for example, bond wires or contact elevations, so-called bumps. The bond pads 12, 13 are comparatively large in order to facilitate such a contact. If the bond pads 12, 13 were placed on the upper side 4 of the mesa, i.e. on the semiconductor element, this would lead to an enlargement of the mesa, which again leads to comparatively high capacitances in the semiconductor element, whereby the semiconductor element would, for example, be unsuitable for high frequencies. The bond pads 12, 13, accordingly, are next to the semiconductor element on the surface of the substrate. The conductor tracks 10, 11 run from the surface 4 across the wall 6 of the mesa 22 down to the surface 7. In practice, semiconductor devices with ever smaller details must be made because of an increasing miniaturization. It is found that miniaturization of known semiconductor devices is accompanied by manufacturing rejects of miniaturized known devices on account of short-circuits between bond pads 12, 13. Figs. 3 and 4 show a semiconductor device according to the invention, reference numerals corresponding to those in Figs. 1 and 2 being used. Fig. 3 is a plan view of one quarter of the semiconductor device and Fig. 4 is a cross-section taken on the line II-II in Fig. 3. The semiconductor device is axially symmetrical around the axis M. According to the invention, the mesa is provided with a depression 50 having a wall 56 and a bottom 57. The insulating layer 8 is also provided on the wall 56 and the bottom 57 of the depression 50. The first semiconductor zones 25 are connected with a bond pad 12 provided on the bottom 57 of the depression 50 of the mesa 22 while the second semiconductor zones 24 are connected with a bond pad 13 provided on the surface 7 of the semiconductor substrate 21 next to the mesa 22. The invention is based on the recognition that short-circuits between the bond pads 12, 13 is due to short-circuits between the conductor tracks 10, 11 connecting the semiconductor zones 24,25 to the bond pads 12, 13 in the case of miniaturization of the known semiconductor device of Figs. 1 and 2. In the known device of Figs. 1 and 2, short-circuits between conductor tracks 10, 11 take place when more than one bond pad 12, 13 lies on the bottom 7 of one depression 5. It is found in practice that the short-circuit takes place on the wall 6 of the mesa 22. Especially the manufacture of the conductor tracks 10, 11 through illumination of a photoresist on the wall 6 for patterning a conductive layer and etching of the conductive layer on the wall, for example by reactive ion etching (RIE), gives rise to problems. When the illumination of the photoresist and the etching take place imperfectly, traces 16 of the conductive layer remain on the wall 6, whereby conductor tracks 10, 11 running across this wall 6 are short-circuited. This happens especially when a groove 17 is created around the mesa structure owing to underetching during manufacture of the mesa. It is found to be very difficult to remove the conductive layer fully from the groove 17. Thanks to the measure according to the invention, only one bond pad 12 is present in the depression 50 and one bond pad 13 on the surface next to the mesa. There will be only one conductor track 11 on the wall 6 of the mesa 22 and also only one conductor track 10 on the wall 56 of the mesa 22, so that short-circuits between the tracks 10, 11 do not arise. The conductor tracks 10, 11 preferably comprise gold. The known device is found to give rise to problems with progressing miniaturization especially when conductor tracks made of gold or an alloy comprising gold are used. This is connected with the fact that layers comprising gold are comparatively difficult to etch because of the noble quality of gold, so that short-circuits between conductor tracks 10, 11 may readily arise. Problems involving short-circuits between layers comprising gold can be prevented through implementation of the invention, while nevertheless the advantages of gold tracks are retained. A track comprising gold has a low resistance, which renders possible short RC times and thus high switching frequencies of the semiconductor element. When the bond pads 12, 13 are also made of the alloy comprising gold, in addition, there will be no corrosion problems between these bond pads and gold bond wires such as those which are generally used for making connections between bond pads and a connection leg of the lead frame. In a further embodiment, the wall 6 of the mesa 22 and the wall 56 of the depression 50 enclose an angle of more than 50° with the surface of the semiconductor substrate (see Fig. 4). This embodiment has the advantage that the mesa structure is sharply defined, so that parasitic capacitances of the semiconductor element remain low. The device according to the invention does not give rise to problems involving short-circuits of conductor tracks 10, 11 even at such an acute angle between the wall 6, 56 and the surface. In a further embodiment, the conductor tracks 10,11 are only patterned on the surface 4 of the mesa 22, the surface 7 of the substrate 21 next to the mesa 22 and the bottom 57 of the depression 50. The conductor tracks 10, 11 are not patterned on the walls 6, 56. Each wall 6, 56, accordingly, is fully covered by a conductive layer. Owing to the special geometry of the semiconductor device according to the invention, no short-circuits between bond pads arise even in this case. Figs. 1 to 4 show that it is possible for a semiconductor element, for example a transistor, to be provided with several separate emitter- and base regions. In such a situation the separate emitter regions may be connected to one emitter bond pad and the separate base regions to one base bond pad. Figs. 3 and 4 show a first embodiment of the invention in which the semiconductor element comprises a mesa in the form of an annular structure with depression 50 inside the structure. The semiconductor device of Figs. 3 and 4 efficiently utilizes the available surface area on the semiconductor body. Fig. 5 is a plan view and Figs. 6, 7 are cross-sections taken on the lines III-III and IV-IV, respectively, in Fig. 5 of a further embodiment of the semiconductor device according to the invention. In this embodiment, the semiconductor element comprises a number of semiconductor sub-elements 100 which are interconnected by dams 150, the semiconductor sub-elements 100 and the dams 150 enclosing the further depression 50. The semiconductor sub-elements 100 have a structure which corresponds to the structure of the semiconductor element of Figs. 1 and 2. The dams are formed from portions of the semiconductor body in that a portion of the semiconductor body was maintained during etching. In contrast to Figs. 1 to 4, Fig. 5 shows the edges of the mesa structure not with two dotted lines 14, 15 but with one dotted line 114. The upper side of the mesa, i.e. the surface 4 of the semiconductor body 1, is accordingly present between the dotted lines 114. The traces 16 which may remain behind on a wall 6, 56 after etching have been left out for clarity in Fig. 5. This embodiment is favourable especially when comparatively small semiconductor elements 100 are required, for example, on account of design requirements such as frequency range and desired power. The dimensions of the bond pads 12, 13 are comparatively great compared with the semiconductor sub-elements 100. The semiconductor sub-elements 100 can have comparatively small dimensions in that the semiconductor element is split up into a number of sub-elements, whereby the design requirements can be complied with, while nevertheless there are no problems involving short-circuits of connection surfaces 12, 13. Figs. 3 to 7 show how the semiconductor element comprises a transistor with base and emitter contacts by contact holes 2, 3 of the semiconductor element. The bond pad 13 for the emitter lies on the surface next to the mesa and the bond pad 12 for the base in the further depression 50. This embodiment has the advantage that the positioning possibility for the bond pad 13 of the emitter zone 24 is wide. Thus the bond pad 13 for the emitter may be placed in such a position around the semiconductor element that the length of a connection between the bond pad 13 and a connection leg of the lead frame is small, while it is also possible in a simple manner to provide more than one bond pad 13, as shown in the embodiment of Figs. 5, 6 and 7. This is especially important for comparatively high-frequency transistors because the length of a connection between the bond pad 13 and a connection leg of the lead frame forms an impedance which adversely affects the high-frequency properties of the transistor; in fact, an increase in the emitter impedance increases the feedback and reduces the transistor gain. When several bond pads 13 are connected in parallel and connected to a connection leg of the lead frame, the impedance is reduced accordingly. No short-circuit takes place between emitter and base of the transistor through the use of the invention. Figs. 5 and 6 show that in a further embodiment of the invention a further contact hole 20 provided with a bond pad to 80 is present on the surface next to the mesa. The further contact hole 20 connects to a collector zone of the semiconductor element. The bond pad 80 serves to make connection with a connection leg of the lead frame. There are two bond pads 13 and 80 for different contact holes 3 and 20 then on the surface 7, one for the emitter region 24 and one for the collector region 21. Each semiconductor subelement in this example is provided with its own bond pad 80. The semiconductor devices of Figs. 3 to 6 may be manufactured by a standard method, for example, as follows. A 1.5 µm thick n-type epitaxial layer 22, which forms a space charge region for the transistor collector, is grown on a highly doped n + -type silicon substrate 21 in a reactor. Then a 50 nm thick p-type layer 23 is grown epitaxially in a reactor. Subsequently, approximately 300 nm plasma oxide is deposited on the surface 4 at a deposition temperature of approximately 400 °C. The plasma oxide is patterned into an etching mask by means of photolithographical techniques, after which mesa structures are realised through etching. Etching starts with anisotropic plasma etching through the p-type region 23, after which the n - -type region 22 is etched away with a KOH etching treatment. Etching is stopped in the highly doped n + region 21. The etching mask is then removed. Subsequently, a plasma oxide 8 of 300 nm thickness is deposited by standard techniques over the entire surface 4 of the semiconductor body 1. This oxide 8 passivates the walls 6, 56 of the mesa 22 where pn junctions between the regions 22, 23 come to the surface. A contact window 2 for the transistor base connection is provided in this oxide 8. The p + -type base contact region 25 is then provided in known manner through the base contact holes 2. Then the contact window 3 for the transistor emitter connection is provided and n + -type emitter regions 24 are provided through these emitter contact holes 3. For the semiconductor device according to Figs. 5, 6 and 7, the contact window 20 for the transistor collector connection is also made. Then conductor tracks 10, 11 of gold are provided in the contact holes 2 and 3 in known manner by the application of a conducting layer and sputter etching, connecting the contact holes 2, 3 to the bond pads 12, 13. For the semiconductor device according to Figs. 5, 6 and 7, bond pads 80 for the transistor collector connection are simultaneously manufactured as well. A connection surface 90 for the collector is provided on a lower side of the semiconductor substrate 21 in the case of the semiconductor device according to Figs. 3 and 4. The semiconductor bodies, of which a large number are manufactured next to one another on a semiconductor wafer, are finally detached from one another by scoring and breaking. The semiconductor body is given a final mounting on a lead frame and enclosed in a housing. The invention is not limited to the embodiments described above. The semiconductor material of the semiconductor body may be germanium, silicon-germanium, GaAs, or some other semiconductor instead of silicon. Techniques other than those described, for example, diffusion instead of implantation or epitaxy, or wet chemical etching instead of plasma etching may alternatively be used. The oxide layer 8 may be TEOS instead of plasma oxide. The doping types may also be the opposites of those in the embodiments, i.e. the first conductivity type is the p-type then. The doping levels may also vary without defracting from the invention. Transistors were described in the embodiments, but the invention may also be applied to other semiconductor elements such as, for example, diodes or thyristors. The semiconductor elements may also be interconnected in the mesa structure so as to form further semiconductor elements in the form of, for example, cascode circuits, or may be combined with, for example, resistor regions and/or capacitors, i.e. the mesa structure then comprises a circuit which is connected by conductor tracks to bond pads in various further depressions.

Claims: A semiconductor device comprising: a semiconductor body (1) consisting of a semiconductor substrate (21) and a mesa (22) formed on the semiconductor substrate (21), said mesa having an upper surface (4) and a wall (6), an insulating layer (8) formed on the upper surface (4), on the wall (6) of the mesa (22) and on the surface (7) of the substrate (21) next to the mesa (22), a semiconductor element comprising first semiconductor zones (25) and second semiconductor zones (24) provided in the upper surface (4) of the mesa (22), contact holes (2,3) formed in the insulating layer (8) on the upper surface (4) of the mesa (22) whereby portions of the semiconductor zones (24,25) of the semiconductor element are exposed, conductor tracks (10,11) provided on the insulating layer (8) which connect the semiconductor zones (24,25) with bond pads (12,13) characterized in that the mesa is provided with a depression (50) having a wall (56) and a bottom (57), the insulating layer (8) is also provided on the wall (56) and the bottom (57) of the depression (50) and the first semiconductor zones (25) are connected with a bond pad (12) provided on the bottom (57) of the depression (50) of the mesa (22) while the second semiconductor zones (24) are connected with a bond pad (13) provided on the surface (7) of the semiconductor substrate (21) next to the mesa (22). A semiconductor device as claimed in claim 1, characterized in that the conductor tracks (10,11) comprise gold. A semiconductor device as claimed in any one of the preceding claims, characterized in that the wall (6) of the mesa (22) and the wall (56) of the depression (50) enclose an angle of more than 50° with the surface of the semiconductor substrate. A semiconductor device as claimed in any one of the preceding claims, characterized in that the conductor tracks (10,11) are only patterned on the surface (4) of the mesa (22), the surface (7) of the substrate (21) next to the mesa (22) and the bottom (57) of the depression (50). A semiconductor device as claimed in claim 1, characterized in that the first semiconductor zones (25) form the emitter zone and the second semiconductor zones (24) the base zone of a bipolar transistor.

Inventors: DEKKER RONALD, GRAVESTEIJN DIRK JAN, MAAS HENRICUS GODEFRIDUS RAFAE, VERSLEIJEN MARTINUS PIETER JOH, DEKKER, RONALD, GRAVESTEIJN, DIRK JAN, MAAS, HENRICUS GODEFRIDUS RAFAEL, VERSLEIJEN, MARTINUS PIETER JOHANNES GERARDUS
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1994
Main classification: H01L23
IPC codes: H01L21, H01L29, H01L23